1. Field of the Invention
The present invention relates to a unidirectional optical amplifier capable of application to many fields, such as electrical engineering, electronic engineering, information engineering and opto-electronics, which amplifies light only in one direction.
2. Prior Art
Prior art methods for effecting optical amplifying include a laser, a traveling-wave tube, and a propagating wave amplification of light due to an interband electron transition.
The laser is a typical opto-electronics element or opto-electronics device for generating light and for amplifying light. The direction of light to be amplified is possibly reversed, and both forward and backward waves may be amplified. Accordingly, when the emitted light is reflected at the surface of a lens, optical fiber, optical disk or the like, and is incident on the laser as return light, this return light has also been amplified. Therefore, the oscillating characteristic and the amplifying characteristic of a laser are deteriorated, and excess noise is generated.
As countermeasures to the above malfunction suggested at present, the typical method is a technique that reentering of return light is generally prevented by providing at an output side of a laser an isolator for passing light only in a single, desired direction. It is, however, only possible to make optical isolators of magnetic material with bulk shape as the main material, and the price thereof is high, so the utility thereof may be limited. On this account, the optical isolator is utilized for fundamental study in the optical field and in optical fiber communication systems of large capacity, but applications requiring small size and low price, the optical disk technology, can not utilize the optical isolator, and thus characteristic deterioration and noise generation due to the return light becomes a technical obstacle in utilizing a laser.
Also, there is a system for performing high-speed information processing by light by integrating the light generating section, amplifier section, modulating section or the like, which utilizes a laser, with each other as an optical integrated circuit. However, in this system, the light returns from a forward section to a backward section, so that a problem emerges that a composition as an optical circuit having a composite function cannot be completed.
Moreover, the traveling-wave tube is a unidirectional electron tube having the highest operable frequency over the upper limit of operable frequency of a usual electron tube having unidirectional electronic functionality or a transistor (about 1000 MHz). This traveling-wave tube makes an electromagnetic wave propagate by using a delayed transmission line made of metal. An electron beam emitted from an electron gun gives energy to this electromagnetic wave and the electromagnetic wave is amplified when the speed of the electron beam and the propagation speed of the electromagnetic wave are coincident with each other. Other electromagnetic wave components traveling in the reverse direction are not amplified. However, the higher the frequency, the shorter the wavelength, so that the upper limit of usable frequency of a traveling-wave tube is determined by manufacturing techniques for the metal of the transmission path. As a result, the traveling-wave tube can not be utilized with frequencies in the range of dozens of GHz or more (wavelengths of several cm or less). Accordingly, fabrication of a traveling-wave tube capable of applying light having a wavelength of less than 1 .mu.m exceeds the limitations of current manufacturing techniques, and is impossible at this time.
Moreover, in the prior art of traveling wave amplification of light by interband electron transition, there have been attempts at unidirectional amplification of light using a semiconductor laser performing generation and amplification of light by electron transitions from conduction band to valence band in a semiconductor and by taking a value of momentum of light, h.beta./2.pi. (where, h is Plank's constant, and .beta. is wave number of light) that may be ignored in most cases because its value is usually small. In this case, since scattering of an electron is extremely large, clear unidirectional amplifying action has not been confirmed.